[R] Draw level lines on the surface of a bivariate function

[Cuvelier Etienne]

Hello,
Is it possible to draw level lines  on the surface of a bivariate function?

In the following example, to draw surface and levels lines for a 
multivariate normal law,
I use persp, trans3d, contourLines and lines,
but if  the lines are correctly drawn, some parts of them are, of 
course, visible
even if they are drawn on a non visible "face".

Any suggestion to avoid this problem ?
Thank you

Etienne

Example :

trans3d <- function(x,y,z, pmat)
{
   tr <- cbind(x,y,z,1) %*% pmat
   list(x = tr[,1]/tr[,4], y= tr[,2]/tr[,4])
}

mean1 = c(2,4)
cov1 = matrix(c(0.7, 0.2,0.2,0.7), ncol=2)

x = seq(0, 5, by = 0.1)
y = seq(0,10, by= 0.1)
z = matrix(nrow = length(x), ncol=length(y))

for(i in 1:length(x))
  for(j in 1:length(y))
  {
    z[i,j]=dmvnorm(c(x[i],y[j]),mean1,cov1)
  }

pmat1 = persp(x, y, z, col= "red",shade = 0.25, border=NA)

lev =contourLines(x,y,z, nlevel = 10)
for(i in 1:length(lev))
  lines(trans3d(lev[[i]]$x, lev[[i]]$y, lev[[i]]$level, pmat1), col="black")


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